박슬래브 연속주조시 탕면 레벨 불안정 발생 원인규명을 위한 수치해석

Abstract

This study introduces a main reason of mold level instability in the thin slab continuous casting process. It is known that mold level instability affects productivity and quality of steel products and, is caused by unsteady bulging. For many years, researchers tried to reveal the main reason of unsteady bulging. However, there are some limitations to clarify this phenomenon. Therefore, goal of this study is to prove the mechanism of unsteady bulging from ideal model and the feature of unsteady bulging from real model. In order to elucidate the cause of mold level fluctuations, the simple finite element model of the thermo-elastoplastic creep model was made using ABAQUS. The behavior of solidified shell between the rolls was calculated and analyzed with fast Fourier transform. The main cause of mold level fluctuation could be categorized two mechanisms at the upper side of strand and at the crater end of the strand. In the upper side of the strand, the solidified shell is enough thin to be deformed by the ferrostatic pressure and bulging is made as steady state. If there are no perturbations such as uneven solidified shell and variation of roll friction or casting speed, the unsteady bulging would not be occurred. If the uneven shell is made during the solidification at the mold or upper region of secondary cooling zone, the unsteady bulging is made between the two rolls. This unsteady bulging give rise to the change of the friction at the rolls, then it makes the variation of casting speed. Because the unsteady bulging and variation of casting speed change the interior volume of the unsolidified liquid of the strand, the meniscus level fluctuated same frequencies of uneven solidified shell, roll pitch and casting speed. In the region of the crater end position, because the solidified shell is enough thick to withstand the ferrostatic pressure, the behavior of the solidified shell is almost rigid body motion. As the unsteady bulged strand moves down to the crater end position and the shell can be withstand the ferrostatic pressure, the profile of the unsteady bulging is remained and conduct as rigid body. If this unsteady profile of the bulging pass the rolls, the interior volume of the unsolidified liquid of the strand also changed with the frequency of the profile of the bulged strand and the meniscus level fluctuated same frequency. This profile can be found at the real plant.